First, they're not casting different people, they're having other characters fill their posts. Second, Doohan, Takei, et al were not "co-stars", despite what they might have said later. Finally, sometimes minor characters are replaced here and there because of illness, scheduling problems, etc. It's well-documented, for example, that a shooting overrun on The Green Berets film prevented Takei from returning to Trek as planned, and thus Sulu was replaced by Chekov during that time.

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“Tact is the ability to tell someone to go to hell in such a way that they look forward to the trip.”
― Winston S. Churchill